The present invention relates to detection apparatuses such as radar or sonar apparatuses for first storing received detection signals defined in a polar coordinate system and coming from the whole horizontal directions into a video memory having an array of memory elements arranged in a rectangular Cartesian coordinate system and then displaying the stored data on an indicator thereof in accordance with a raster scanning method. More particularly, the invention relates to a radar apparatus or like apparatuses such as sonars and a method for writing received data into a memory, which are capable of displaying a target as an image so that the target echo is distinguishable from clutter.
FIG. 1 shows a block diagram of a conventional radar apparatus.
A radar antenna 1 is rotated in a horizontal plane at one time period and successively radiates pulses of radio waves at another period and receives echoes of radio waves reflected by targets. A receiver circuit 2 detects and amplifies signals received by the radar antenna 1. An A/D converter 3 converts an analog signal outputted by the receiver circuit 2 into a digital signal. A primary memory 4 stores signal data for one sweep resulting from the analog to-digital conversion in real time, and is used as a buffer for writing the one-sweep data into a succeeding-stage video memory 6 by the time when echo signal data resulting from a next transmission are written thereinto.
A coordinate converter 5 generates addresses representative of pixels of the video memory arrayed in Cartesian coordinates successively from the center coordinate as a start address to outward directions based on, for example, both an antenna angle xcex8 with respect to the heading direction of the ship and a read position of the primary memory 4 from which a signal is read out. The coordinate converter 5 is constructed by hardware that performs operations in accordance with the following equations:
X=Xs+rxc2x7sin xcex8
Y=Ys+rxc2x7cos xcex8
Where X and Y are addresses representative of a pixel of the video memory;
Xs and Ys are addresses representative of the center of the memory;
r is the distance from the center; and
xcex8 is the angle for coordinate conversion.
The video memory 6 has enough capacity to store echo data for display received during one rotation of the radar antenna 1. A display unit 7 is raster-scanned by a display control unit (not shown). The signals stored in the video memory 6 are read at a high speed in synchronism with the raster scanning so that the received signals are displayed in brightness or different colors depending on the strength thereof.
With the conventional radar apparatus constructed as explained in the foregoing, it has been necessary to appropriately adjust GAIN, STC and FTC controls and the like so that targets are displayed as distinguished from clutter such as sea clutter or rain/snow clutter. In particular, in such cases as the difference between target echoes and clutter in signal strength is small, there will be required a subtle adjustment. Operations are performed for searching and determining an optimal value by comparing an image obtained before an adjustment made and another image obtained after the adjustment performed to control the amount of the adjustment for determining the optimal value.
However, with the foregoing conventional apparatus, there has been such a problem that it takes a long time to complete the adjustment, since images are updated only on the sweep line in line with an antenna pointing direction, and hence the confirmation of an adjustment result will not be done until the rotating sweep line comes up to a straight line extending over the measuring point, and further since the time required for one rotation of the antenna is normally 2 to 3 seconds. Further, if an optimal adjustment value provided to another target is different from the other ones, the adjustment operation is required to be performed for each target. Thus, troublesome adjustment operations should frequently be done to observe displayed images, resulting in a problem that the apparatus is very inconvenient to be used in terms of operability and response. This problem has been found not only in radar apparatuses but also in like apparatuses (e.g., sonars) in which received data defined in a polar coordinate system are successively converted to corresponding ones in a Cartesian coordinate system and stored with rotation of the sweep line. An object of the present invention is to provide a radar and like systems, and a method for writing received date into a memory, which are capable of displaying an image of a target so that the image is readily distinguished from clutter, by reducing the level of image data and simultaneously storing received data into the video memory through coordinate conversion, without any adjustments having been done in prior art apparatuses.
A radar apparatus according to the present invention comprises a video memory for storing signals obtained by coordinate conversions successively converting received data from a polar coordinate system to a rectangular Cartesian coordinate system along with the sweep rotation, and subtraction means for subtracting a constant value from data supplied from memory elements in an area set on the video memory in parallel with writing data into the video memory with the coordinate conversion.
The subtraction means subtracts the constant value from pixel data of the set region so that images within the set region are seen as being gradually disappeared independently of images displayed with the sweep rotation. Thus, the images in the set region can be observed in the same way as images displayed by an adjustment for gradually reducing the gain during one rotation of sweep which was made in the conventional device. As a result, it will be possible to readily display the image of a target as distinguished from clutter even without performing manual fine adjustments.
Also, the radar apparatus according to the invention, further comprises FIRST detection means for detecting an access as a FIRST access (hereinafter referred to as FIRST) that a pixel of the video memory is accessed for a first time at the coordinate conversion during one sweep rotation. The subtraction means performs subtractions at times other than FIRST detections.
In the coordinate conversion, received data are scattered, geometrically, denser in central portions of the coordinate and sparser in peripheral portions thereof. Thus, the nearer the address of a pixel in the video memory to the center thereof, the more a data amount corresponding to the one pixel of the memory. In this case, data to be written into one pixel of the video memory is selected from among a plurality of received data. When writing-over operations are merely performed in one pixel, the last written data is stored therein. While, when a data having the largest value is selected from among the received data and written thereinto, FIRST detection and MAX process are executed. The FIRST detection is an operation of detecting that an access to a pixel of the video memory is made for the first time during one sweep rotation. The MAX process is an operation that when a FIRST is detected, data received at that time is written into a memory element, and that when a FIRST is not detected (i.e., at a second and following accesses), the current received data and data having been written in the pixel are compared with each other in magnitude to write a data having larger value over the previously written data. Thus, there are methods such as the one in which a new received data is written over a previously stored one into one pixel at a coordinate conversion, or another one in which a data having the largest value is written thereinto both by the FIRST detection and the MAX process as explained above. In either case, since only one piece of data can be written into one pixel in the video memory, only one access is required in order to satisfy the condition that one piece of data is written into one pixel. Accordingly, in the present invention, when a FIRST detection is made, an access to the pixel of the video memory is made to write a received data. When a FIRST is not detected, accesses for coordinate conversion to the video memory are not executed. With such an arrangement, there will be provided a period during which no access for coordinate conversion is made to the video memory during the coordinate conversion period. Therefore, during this period, the subtraction process on pixel data in the region is performed by the subtraction means.
With the present invention, it is also possible that a radar apparatus comprises LAST detection means for detecting, as a LAST access (hereinafter referred to simply as LAST), an access that a pixel of the video memory is accessed for the last time at the coordinate conversion during one round of sweep, wherein the subtraction means performs subtraction operations at times other than the LAS detection.
That is, the coordinate conversion is executed only at the time of a LAST detection instead of the time of the FIRST detection. Therefore, at a coordinate conversion in which a LAST is not detected on one pixel, the operations by the subtraction means will be performed.
Also, in the present invention, the region may be arbitrarily designated by region designating means. With such an arrangement, it becomes possible, for example, to designate a region of interest on the screen by a cursor or the like and display the image of a target as distinguished from clutter within the region.
Since the ratio of the FIRST access period or the LAST access period to the whole period becomes larger with increase of the sweep rotation speed, the period over which the entire region is accessed for subtraction will be eventually shortened so that the gradual image disappearance would no longer be fulfilled sufficiently. In such a case, the size of the area specified is reduced and is freely moved on the screen to achieve the same object.